Testing spline tool

[0HyperCube]

Testcases:

• Works when the viewport is zoomed/panned/tilted
• Works when the artboard has an offset (currently broken Spline tool on offset artboard #2484)
• Works when the artboard has a transform before it
• Closed splines can be drawn by clicking on the start. The tool should stop drawing. The second derivative of the curves should be continuous.
• Continuing to draw from a previous spline
• Continuing drawing when the other spline is transformed and in a group (currently broken Spline tool extending a transformed spline #2482)
• Continuing to draw from some other path